Various redox mediators have been evaluated and characterized in dye-sensitized solar cells (DSCs): tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS), a series of dithioates, and cobalt(II/III) tris(bipyridyl) complexes. These mediators were characterized with a range of photoelectrochemical techniques, including cyclic voltammetry, current-voltage characteristics, incident photon-to-current efficiency, transient absorption, and impedance spectroscopy. Important contributions to the field include: (i) identifying that TMTU cannot be used with conventional ruthenium dyes due to inefficient regeneration; (ii) improving power conversion efficiency (PCE) in TMTU based DSCs; (iii) developing a new approach for accurately quantifying electron diffusion length in DSCs when it is shorter than the TiO2 film thickness; (iv) clarifying the importance of steric hindrance on dye molecules in determining recombination kinetics; (v) identifying a dye-dependent recombination pathway that depends on the steric bulk of the dye; (vi) improving PCE in ruthenium dye DSCs utilizing the cobalt(II/III) tris(bipyridyl) mediator by incorporating alkylphosphonic acid coadsorbents into the dye monolayer.